Motion picture photographic films have long used a carbon black-containing layer on the backside of the film. This backside layer provides both antihalation protection and antistatic properties. The carbon black is applied in an alkali-soluble binder that allows the layer to be removed by a process that involves soaking the film in alkali solution, scrubbing the backside layer, and rinsing with water. This carbon black removal process, which takes place prior to image development, is both tedious and environmentally undesirable since large quantities of water are utilized in this film processing step. In addition, in order to facilitate removal during film processing, the carbon black-containing layer is not highly adherent to the photographic film support and may dislodge during various film manufacturing operations such as film slitting and film perforating. Carbon black debris generated during these operations may become lodged on the photographic emulsion and cause image defects during subsequent exposure and film processing.
After removal of the carbon black-containing layer the film's antistatic properties are lost. Undesired static charge build-up can then occur on processed motion picture film when transported through printers, projectors or on rewind equipment. Although these high static charges can discharge they cannot cause static marks on the processed photographic film. However, the high static charges can attract dirt particles to the film surface. Once on the film surface, these dirt particles can create abrasion or scratches or, if sufficiently large, the dirt particles may be seen on the projected film image.
In U.S. Pat. No. 5,679,505, a motion picture print film is described which contains on the backside of the support, an antistatic layer and a protective overcoat. The protective overcoat is comprised of a polyurethane binder and a lubricant. The polyurethane binder has a tensile elongation to break of at least 50% and a Young's modulus measured at 2% elongation of at least 50000 lb/in.sup.2. The tough, flexible overcoat has excellent resistance to abrasion and scratching during manufacture, printing, and projection, while acting as an effective processing barrier for the underlying antistat layer.
However, post-processing tar deposits and stain have been a problem with protective overcoats in motion picture film. This tar is derived mostly from polymeric oxidized developer. The present invention relates to eliminating tar pickup during processing by providing a very thin topcoat over the polyurethane layer. The topcoat is obtained by the coating and drying of a coating composition comprising gelatin-coated, latex particles. The topcoat is effective at coverages so low that the excellent physical properties conferred upon the support by the polyurethane are retained.
In copending commonly-assigned United States Patent Application Ser. No. 08/856,711 now U.S. Pat. No. 5,786,134, is described a motion picture film having a topcoat that resists processor tar pickup. The topcoat contains at least 20 percent by weight of a hydrophilic colloid such as gelatin.
It would be desirable to provide a stain resistant topcoat of that resists processor tar stain, is readily manufactured and provides good physical properties.